The invention relates to a process for producing impregnable fine mica tapes with a built-in accelerator which after winding onto the conductors of the windings of electrical machines are impregnated with a solvent-free impregnation resin and afterwards are cured under the action of heat, the fine mica film being sprinkled with a powder epoxy-resin systems and the side of the fine mica film which has been sprinkled with the powder epoxy being cemented to the carrier material under the influence of pressure and heat.
To insulate winding elements such as for example conductor rods in rotating high voltage electrical machines, the impregnation technique based on vacuum pressure impregnation has become very popular in recent years. In doing so the winding elements for construction engineering reasons are made either as performed coils or conductor bars, preferably Roebel bars. These winding elements are provided with mica-containing main insulation and are further treated in a vacuum pressure impregnation process. Here epoxy resins, preferably low-viscosity, solvent-free resin systems are used as the impregnation resins.
At this point they impregnate the mica-containing main insulation formed from several winding layers so that cavities which can cause partial discharges between the winding layers are completely filled.
The winding layers of the main insulation are formed by mica-containing insulating tapes. They consist of a fine mica film which is cemented to a carrier material by means of a powdered adhesive resin.
The carrier material should be porous so that the solvent-free impregnation resins can impregnate it. To accelerate the cross-linking reaction of the impregnation resin during the curing process the fine mica films used are also impregnated with an accelerator.
The powder epoxy-resin systems used are preferably resin systems which are free of curing agent and which have complete solubility in the impregnation resin and react into it during the curing process.
It is still possible with these accelerator-containing insulating tapes using powder epoxy-resin systems which are free of curing agent to impregnate insulating wall thicknesses up to roughly 3.5 mm completely and in a relative short impregnation time.
But it has been shown that with increasing size and power of generators relatively high insulating wall thicknesses become necessary which likewise are to be completely impregnated with solvent-free impregnation resin during the impregnation phase.
When using the aforementioned accelerator-containing mica tape which has a powder epoxy-resin systems which is free of curing agent it has however been shown that for high insulating wall thicknesses of more than 3.5 mm they are no longer completely impregnated with the impregnation resin. The unimpregnated winding layers are faults in the insulation system and therefore cause possible later failure.
Therefore the object of the invention is to eliminate the defects of these accelerator-containing mica tapes, defects known in the past from the prior art. As claimed in the invention, a powder epoxy-resin system which contains a curing agent and which cures very quickly at an elevated temperature when the fine mica film is cemented to the carrier material is incorporated into the initially named process for producing impregnable fine mica tapes with a built-in accelerator.
It was surprisingly found that by using a powder epoxy-resin system which contains a curing agent as the adhesive resin in accelerator-containing mica tapes likewise high insulating wall thicknesses exceeding 3.5 mm can be completely impregnated.
This technical effect is achieved by the fact that the powder epoxy-resin system which contains a curing agent cross links and represents a spot bond which is insoluble in the impregnation resin when the carrier material is cemented to the fine mica film. Since the spot bond wets only a small percentage of the impregnable surface of the carrier material, the remaining unwetted surface area can be easily impregnated. In addition, the curing of the cement prevents an undesirably fast reaction, as is the case in resin systems which are free of the curing agent and those which are soluble in the impregnation resin at roughly 60xc2x0 C., between the powder enamel resin and impregnation resin during the impregnation phase so that even high insulating wall thicknesses are completely impregnated with impregnation resin.
Furthermore it is suggested as claimed in the invention that the powder epoxy-resin system which contains a curing agent has a bisphenol-A epoxy resin and a phenol novolak epoxy resin and that the curing agent used is an aminic curing agent, preferably a primary and/or secondary aliphatic resin.
Preferably the aminic curing agent is used in a minimum amount so that during curing a complete reaction of the amine groups with the oxiran groups of the epoxy resin is guaranteed.
The resulting compound thus has no primary amine functions, is insoluble in the impregnation resin, and thus it cannot exert an accelerating effect on it.
Other advantages of the process as claimed in the invention consist in that the carrier material used is a glass fabric, a formed fabric or a plastic film and that the fine mica film used is preimpregnated with an accelerator, preferably zinc naphthenate.